1. Field of the Invention
The present invention relates to semiconductor devices in general and to a three dimensional TFT array in particular.
2. Discussion of Related Art
As integrated circuits and computers have become powerful, new applications have arisen that require the ability to store large amounts of data. Certain applications require a memory with the ability to write and erase data and the ability to store data in a nonvolatile manner. There are many applications which can be enabled by bringing the price per megabyte of semiconductor memory down well below a dollar (US) per megabyte so that it becomes price competitive with, for example: (1) chemical film for the storage of photographic images; (2) Compact Disks (CDs) for the storage of music and textual data for distribution; (3) Digital Versatile Disks (DVDs) for the storage of video and multi-media materials for distribution; and (4) Video Tape and Digital Audio and Video Tape for the storage of consumer audio and video recordings. Such memories should be archival and non-volatile in that they should be able to withstand being removed from equipment and all sources of power for a period of up to about 10 years with no significant degradation of the information stored in them. Such a requirement approximates the typical longevity for CDs, DVDs, magnetic tape and most forms of photographic film.
Presently, such memories are formed with electrically erasable nonvolatile memories such as flash memories and EEPROMs. Unfortunately, these devices are typically fabricated in a single crystalline silicon substrate and therefore are limited to two-dimensional arrays of storage devices, thereby limiting the amount of data that can be stored to the number of devices that can be fabricated in a single plane of silicon.
It has also been known to fabricate nonvolatile memories that employed trapped charge in a dielectric layer. Typically, electrons are trapped in a layer of silicon nitride by, for instance, tunneling a current through the nitride layer. The silicon nitride is formed between a gate insulated from the channel of a field-effect transistor. The trapped charge shifts the threshold voltage of the transistor and thus, the threshold voltage is sensed to determine whether or not charge is trapped in the nitride layer. See U.S. Pat. No. 5,768,192 for an example of such memories.
U.S. Pat. No. 5,768,192, issued to B. Eitan, and the technical article entitled “NROM: A Novel Localized Trapping, 2-Bit Nonvolatile Memory Cell” by B. Eitan et al. in IEEE Electron Device Letters, vol. 21, No. 11, November 2000, pp. 543-545 teach a nonvolatile semiconductor memory cell which uses asymmetrical charge trapping in the nitride charge storage layer of the Oxide-Nitride-Oxide (ONO) stack to store two bits in one cell. The cell is written by hot electron injection into the charge storage layer above the drain junction. The cell is read in the opposite direction to which it was written, i.e., voltages are applied to the source and gate, with the drain grounded. The memory cell is constructed in a p-type silicon substrate. However, this silicon-oxide-nitride-oxide-silicon (SONOS) ITC memory is arranged in an NOR Virtual Ground Array with a cell area of 2.5F2 per bit, where F is the minimum feature size. This cell area is larger than desirable, and leads to a less than optimum cell density.
Prior art negative-resistance devices are also known. These devices were discovered around 1972 and are described in Thin-MIS-Structure Si Negative-Resistance Diode, Applied Physics Letters, Volume 20, No. 8, beginning on page 269, 15 Apr. 1972. The device described in the article is a junction diode, such as diode 5510 of FIG. 96 and a thin oxide region disposed on the n-type region of the diode, such as the oxide region 5511 of FIG. 96. The device provides a switching phenomenon exhibiting a negative-resistance region as shown in FIG. 97. Note as the potential on the diode is increased in the diode's forward direction, little conduction occurs until the voltage first reaches the voltage shown as point 5512 at which point the device exhibits a negative-resistance. From there the device exhibits a somewhat diode-like characteristic as shown by the segment 5513 in FIG. 97. This switching characteristic is used to fabricate static memory cells (flip-flops) such as shown in U.S. Pat. Nos. 5,535,156 and 6,015,738. Additionally, the basic operation of this device is described in Sze's, The Physics of Semiconductor Devices, (2nd edition, Chapter 9.5, pp. 549-553), although this explanation may contain an error in its discussion in polarity.
The device of FIG. 96 comprises a PN junction diode and a thin oxide region. When the diode is forward biased, initially very little current flows because the diode junction voltage is a fraction of the applied voltage, with the balance of the voltage drop across the n− region and oxide region. Holes injected into the n− region from the p region are sufficiently low in number that the tunneling current through the oxide (despite the unfavorable barrier to the hole flow) allows the n− region to remain an n-type region. Similarly, any holes generated within the depletion region are able to pass through the thin oxide while any generated electrons are swept across to the p region and out of the anode contact.
As the applied forward voltage increases, the n− region begins to deplete at the interface with the oxide just as in a normal MOSFET as the threshold voltage is approached. At a high enough voltage, this depletion region extends all the way to the junction to produce punch-through, resulting in a significant injection of holes from the p region into the n− layer. The holes cannot flow well through the oxide and consequently build up near the surface. This causes the n-region to invert more strongly near the oxide interface, and increasing the voltage drop across the oxide, recalling that V=Q/C. The electron tunneling current through the oxide rises by a super-exponential factor, increasing the forward bias across the diode and the current. At the same time holes flood the n− region, raising its conductivity and reducing its voltage drop. Since the voltage across the diode is relatively small (and changes little, even for large changes in current) a large reduction in the n− voltage drop reduces the voltage across the entire structure dramatically (assuming a suitable series resistance in the circuit to avoid device rupture). Thus, the regenerative action of the foregoing description causes a rapid increase in current, accompanied by a rapid decrease in voltage. It is this negative-resistance region that has been exploited to make the SRAM cells described in the above referenced patents.
At higher current levels, the device behaves essentially as an ordinary forward biased diode as most of the voltage is ultimately dropped across the PN junction. Overall, the V-I characteristics of the structure are shown in FIG. 97 with the slope of the segment 5513 being determined in large part by the series resistance coupled to the structure of FIG. 96.
When reverse biased, the diode is in its blocking state and the only current that flows through the oxide is electron leakage current. The reverse junction voltage is a fraction of the applied voltage because some is dropped across the oxide region. It should be noted that electrons carry current through the oxide region in both reverse bias and in a strong forward bias.
Another type of prior art memory device is disclosed in the technical article entitled “A Novel Cell Structure for Giga-bit EPROMs and Flash Memories Using Polysilicon Thin Film Transistors” by S. Koyama in 1992 Symposium on VLSI Technology Digest of Technical Papers, pp. 44-45. As shown in FIG. 98, each memory cell is a “self-aligned” floating gate cell and contains a polycrystalline silicon thin film transistor electrically erasable programmable read only memory (TFT EEPROM) over an insulating layer. In this device, the bit lines extend in the direction parallel to the source-channel-drain direction (i.e., the bit lines extend parallel to the charge carrier flow direction). The word lines extend in the direction perpendicular to the source-channel-drain direction (i.e., the word lines extend perpendicular to the charge carrier flow direction). The TFT EEPROMs do not contain a separate control gate. Instead, the word line acts as a control gate in regions where it overlies the floating gates.
The layout of Koyama requires two polycide contact pads to be formed to contact the source and drain regions of each TFT. The bit lines are formed above the word lines and contact the contact pads through contact vias in an interlayer insulating layer which separates the bits lines from the word lines. Therefore, each cell in this layout is not fully self-aligned, because the contact pads and the contact vias are each patterned using a non-self aligned photolithography step. Therefore, each memory cell has an area that is larger than desirable, and leads to a less than optimum cell density. The memory cell of Koyama is also complex to fabricate because it requires the formation of contact pads and bit line contact vias. Furthermore, the manufacturability of the device of Koyama is less than optimum because both bit lines and word lines have a non-planar top surface due to the non-planar underlying topography. This may lead to open circuits in the bit and word lines.
The Virtual Ground Array approach to crystalline silicon non-volatile memories has also been known for some time and is an elegant way of aggressively reducing memory cell size. Turning now to FIG. 99, the basic approach utilizes a cross point array 5610 of bitlines in buried n+ diffusion 5612 within a single crystalline silicon p-type substrate 5614 and wordlines formed of polysilicon rails 5616 disposed over the substrate 5614. A transistor is formed from adjacent bitlines 5612 and a p-type channel region 5618 disposed between the adjacent bitlines 5612. A layer of gate oxide 5620 insulates the floating gates 5622, which lie above the channels 5618 and are formed of, for example, polysilicon. An upper dielectric layer 5624 insulates the floating gates 5622 from polysilicon wordlines (WLs) 5616.
“Virtual Ground” refers to the fact that there is no dedicated ground line in the array. Whenever a cell is chosen for read or program, a pair of buried n+ bitlines (BLs) is the source and drain with the source grounded. For example, to select the cell 5624 outlined in FIG. 100, BL(k) and BL(k+1) would be selected as the source and drain (or vice versa) and WL(j) would be selected as the control gate of the device. In one approach, all of the bit lines to the left of BL(k) as shown in FIG. 100 would be held at the same potential as BL(k) and all of the bit lines to the right of BL(k+1) would be held at the same potential as BL(k+1) so that source-drain current would only flow (for read and programming) in the selected cell (all other WLs being grounded).
In all of these approaches, the charge storage medium is a conducting floating gate made of doped polysilicon. By hot electron injection programming (the method of choice in all classic EPROM (erasable programmable read only memory) and single transistor Flash memory cells), electrons are injected onto the floating gate thus changing the threshold voltage of the inherent MOS transistor.
The above discussed SONOS (polysilicon-blocking oxide-nitride-tunnel oxide-silicon) charge trapping approach has reemerged as a viable candidate for non-volatile MTP memories arranged in a virtual ground array structure 5626, as shown in FIG. 101. The array includes n+ buried bitlines 5612 disposed in a single crystalline silicon substrate 5614. An ONO (oxide-nitride-oxide) dielectric stack 5628 insulates bitlines 5612 from polysilicon wordline 5630. The hot electrons are injected into the ONO dielectric stack 5628 near the drain edge during programming where charge is trapped in the nitride layer. Two bits can be stored per memory cell utilizing this approach because hot electrons are injected into the ONO dielectric stack at the programming drain edge. Since the nitride charge storage medium does not laterally conduct, the charge stays where it was injected. Trapped charge near the source of a transistor has a large effect on the transistor's threshold voltage while trapped charge near the drain has little effect on threshold voltage. Accordingly, individual charge zones on either side of the ONO layer may be written and read by simply reversing the drain and source connections for the cell. When the cell is programmed, charge is injected at the zone closest to the drain. If source and drain are reversed for the same cell, another charge may be injected into the same cell but at the “other” drain. Both sides can also be read, thus two bits per cell may be stored and retrieved.
The above described prior art devices are relatively expensive because their density is not optimized.